This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No. PCT/FI97/00836 which has an International filing date of Dec. 31, 1997 which designated the United States of America.
The present invention relates to a hydroxyalkylated starch ester which is a hydroxypropyl starch ester, comprising an ester group derived from acetic acid, propionic acid or butyric acid or from a mixture thereof, the molar substitution of the hydroxypropyl group being 1.4 at the most and the degree of substitution of the ester group at least 1.
The invention also relates to a thermoplasticized starch component containing 90 to 60% by weight of a hydroxypropyl starch ester whose hydroxypropyl group has a molar substitution of up to 2 and whose ester group has a degree of substitution of at least 1, and 10 to 40% by weight of a plasticizer. This thermoplasticized starch may also have additives known as such within the fields of polymer and plastics technology. The invention also relates to a process for preparing a hydroxyalkylated starch ester wherein the starch-containing base is hydroxypropylated and esterified, characterized in that the hydroxypropylation is performed in an aqueous alkanol medium containing 10 to 95% by weight of a lower alkanol, 5 to 90% by weight of water and 0.1 to 10% by weight of a catalyst.
The current emphasis on an environmentally friendly attitude and green values is opening up new markets for products based on renewable natural resources. Such trends can be seen within the sectors of, e.g. the packaging industry, the sanitary industry and the glue industry, where recyclability, reuse, compostability, biodegradability and lack of environmental strain are demands of today. The trend of replacing products based on petrochemistry by processed biopolymer products is also emphasized. Starch and its derivatives constitute a particularly interesting initial material for the preparation of biodegradable polymer products.
Of industrially usefull starch derivatives, particularly starch esters and ethers may be mentioned, as well as mixed ester-ethers of starch, containing both ester and ether groups. The latter are represented by hydroxyalkylated starch esters which have been described, among others, in U.S. Pat. Nos. 4,997,581, 4,193,989, 4,041,179, 4,035,572 3,883,666, 5,360,845, 4,383,988, 4,067,824, 4,035,572 and 3,824,085.
The most important among hydroxyalkylated starch esters are hydroxypropylated derivatives because they can be prepared in room temperature using a liquid material, propylene oxide (1,2 epoxy propane) which reacts easily with the hydroxy groups of starch whereby an ether of starch and 1- and 2-propoxy groups is formed. Other hydroxyalkylated derivatives can be prepared from other corresponding epoxy alkane compounds. Unreacted hydroxyl groups can be esterified further by means of an esterifying agent whereby the corresponding acyl groups are obtained. As the alkylene oxide groups in connection with alkoxylation easily react with one another forming an oligomer, several alkylene oxide residues may be attached to each etherified hydroxyl group. Therefore, the number of hydroxy alkyl groups is given as the molar substitution degree (MS) of the glucose unit, the MS typically varying between 3 and 6. The number of starch ester groups in the glucose unit, that is, the degree of substitution (DS), is 3 at the most.
In the prior art, hydroxypropyl starch acetate has mainly been used as a basic material for chewing gum (e.g., U.S. Pat. No. 4,041,179, U.S. Pat. No. 4.035.572 and U.S. Pat. No. 3,883,666) and as a gelling agent or, rather, an agent which increases the viscosity of the blend in suntan lotions, perfumes, antiperspirants or in organic solvents in general (e.g. U.S. Pat. No. 3,824,085, U.S. Pat. No. 4,067,824, U.S. Pat. No. 4,383,988 and U.S. Pat. No. 4,193,989). Typically, hydroxypropyl starch acetate can be used as a basic material for chewing gum, the DS thereof being 0.5-0.92 and its MS=3-6 (U.S. Pat. No. 4,041,179), or DS=1 -2.5 (1.3-1.65) and MS=3-6 (4.4-4.5) (U.S. Pat. No. 3,883,666).
In the above-described uses the hydroxypropyl starch acetate is typically prepared by allowing the starch dispersed in an organic solvent (preferably toluene) to react with propylene oxide in the presence of NaOH, whereafter the reaction is continued with an acetic acid anhydride. Next, excess toluene is removed by distillation from the toluene/rubber blend, the final product is dissolved in chloroform or ethanol and the purified rubber is precipitated from hexane or water and dried.
By way of summarizing the prior art solutions it can be observed that known hydroxy-alkylated starch esters are not suitable for use as raw materials of compostable starch bioplastics; for that use, they are too glutinous and difficult to mould. In addition, the above-described methods for preparing hydroxypropyl starch acetate are not particularly environmentally friendly, among other reasons because carcinogenic organic solvents are used.
The present invention aims at removing the drawbacks of the prior art and at achieving a new starch derivative as a raw material for starch plastics. A further aim of the invention is new starch derivative as a raw material for starch plastics. A further aim of the invention is to provide a new method for the preparation thereof and to achieve new thermoplasticized starch components and compositions. The invention also relates to the use of these products.
The invention is based on the concept of hydroxyalkylating the starch to a relatively low molar substitution degree and esterifying it to as high a substitution degree as possible, whereby a thermoplasticizable hydroxyalkyl starch ester is obtained which behaves in the manner of plastics. According to the invention, the MS of a hydroxyalkylated starch ester is smaller than 2 and its DS is greater than 1; thus, the MS of hydroxypropyl starch acetate is typically 0.05 to 1.2 and its DS is 1.5 to 3. The hydroxyalkylated starch esters can be mixed with plasticizers whereby a thermoplasticized starch component is obtained containing 90 to 60% by weight of a starch derivative and 10 to 40% by weight of a plasticizer. A useful composition may be formed of the starch component by admixing it with auxiliaries and additives known as such.
By means of the invention, it is possible to obtain a new process for preparing an esterified starch ether used as a raw material for starch-based bioplastics. It is characteristic of the process that starch is etherified in an aqueous alkanol medium in the presence of a catalyst, whereafter the product is filtered and the alcohol is evaporated. After the evaporation of alcohol the hydroxypropyl starch is esterified, washed and dried.
In more detail, the thermoplasticized starch derivative according to the invention is a hydroxypropyl starch ester, comprising an ester group derived from acetic acid, propionic acid or butyric acid or from a mixture thereof, the molar substitution of the hydroxypropyl group being 1.4 at the most and the degree of substitution of the ester group at least 1.
The thermoplasticized starch component according to the invention then, is characterized as containing 90 to 60% by weight of a hydroxypropyl starch ester whose hydroxypropyl group has a molar substitution of 2 at the most and whose ester group has a degree of substitution of at least 1, and 10 to 49% by weight of a plasticizer. This thermoplasticized starch may also have additives known as such within the fields of polymer and plastics technology.
The process according to the invention for preparing a starch derivative is a process for preparing a hydroxyalkylated starch ester wherein the starch-containing base is hydroxypropylated and esterified, characterized in that the hydroxypropylation is performed in an aqueous alkanol medium containing 10 to 95% by weight of a lower alkanol, 5 to 90% by weight of water and 0.1 to 10% by weight of a catalyst.
Considerable benefits are provided by the invention. Thus, the above-described raw materials of the starch-based bioplastics and dispersion according to the invention are mainly based on renewable natural resources and are biodegradable/compostable. The starch component may be derived from any native starch; it need not be, for instance, a starch rich in amylose.
The derivative described herein, when thermoplasticized, has excellent elasticity, melt strength and adhesion properties, and it is easily thermoplasticized. In this respect, it is clearly more advantageous than starch esters. Thus, it is characteristic of the plasticized derivative that its elasticity/elongation is significantly better than e.g. the corresponding properties of a thermoplasticized starch ester. In addition, the adhesion properties of the thermoplasticized starch component, i.e. the esterified starch ether, onto paperboard, paper and other surfaces such as glass, are clearly better than those of a thermoplasticized starch ester.
The plasticized starch derivative can be used within fields of application typical of polymers which are elastic and strong in the molten state, such as in the coating of board and paper, in blown films and films, and it can be used to manufacture injection moulded products. It is possible to process the plasticized starch derivatives and to turn them into products by any method known as such within the polymer technology. In addition, one field of use covers fibres and non-woven fabrics.
According to a preferred embodiment, a dispersion may be produced of the esterified starch ether by plasticizing it and by dispersing the plasticized derivative into water by means of dispersion auxiliaries known as such. No solvents which need to be evaporated are needed for the formation of the dispersion; instead, the dispersion can be performed by means of a melt-processing apparatus. The films formed of the dispersion are water-repellent and can be used to improve the water resistance of paper or board by at least 40 to 50%. The new polymer dispersions may be used for coating paper or board, as a primer or as a component in labelling adhesives or paint. They are also suited for the production of hydrophobic cast films and as binders in materials based on cellulose fibres.
In the following, the present invention is examined in more detail by the aid of a detailed description and a few working examples.
Of the annexed figures, FIG. 1a is a scanning electron microscope picture of the fracture surface of a starch/acetate/cellulose fibre composite (C) and FIG. 1b is a corresponding picture of the fracture surface of an HPS/acetate/cellulose fibre composite (D) (204xc3x97magnification).
In the context of the present invention, the term xe2x80x9cthermoplasticized starch componentxe2x80x9d is used to refer to a combination which can be prepared from an esterified starch ether and which can be moulded at moderate temperatures and pressures by means of plastic processing apparatuses.
The thermoplasticized/thermoplasticizable starch derivative according to the invention may be based on any native starch having an amylose content of 0 to 100% and an amylopectin content of 100 to 0%. Thus, the starch may be derived from barley, potato, wheat, oat, peas, maize, tapioca, sago, rice, or a similar tuber-bearing or grain plant. It can also be based on starches prepared from said native starches by oxidizing, hydrolyzing, cross-linking cationizing, grafting, or by means of an enzymatic treatment.
The starch derivative according to the invention is advantageously an esterified starch ether, preferably a hydroxy propyl starch acetate whose hydroxy propyl groups have a molar degree of substitution (MS) of 1.4 at the most and the ester group has a degree of substitution (DS) of at least 1.5. The MS is particularly advantageously about 0.05 to 1, preferably about 0.3 to 0.7, and the degree of substitution in the ester group is 2.5 to 3.
According to a particularly preferred embodiment, hydroxy propyl starch acetate is prepared whose hydroxy propyl groups have a molar substitution degree of 0.4 to 0.6, the acetate groups having a substitution degree of 2.5 to 2.95.
The ester groups of the derivative may be derived from one or several aliphatic C2-24 carboxyl acids, anhydrides of these acids or acid chlorides or similar reactive derivatives. Thus, the carboxyl acid component in the ester may be derived from a lower alkane acid, such as acetic acid, propionic acid or butyric acid or a mixture thereof. The carboxyl acid component may, however, also be derived from a saturated or an unsaturated native fatty acid. As examples of these, palmitinic acid, stearic acid, oleic acid, linoleic acid and mixtures thereof may be mentioned. The ester may also be composed of both long- and short-chain carboxyl acid components. As an example, a mixed ester of acetate and stearate may be mentioned.
The starch derivative according to the invention is prepared by a two-phase method by first hydroxyalkoxylating one of the above-described starches by means of a corresponding etherifying agent in the presence of a catalyst, whereafter the hydroxyalkyl starch obtained is esterified by means of an esterifying agent.
In connection with the invention it has been found that the desired low MS value of the product is attained by performing the hydroxypropylation in alkanol because this does not essentially gel the starch, whereby the reaction takes place without degradation of the granular structure of the starch.
Thus, in connection with the hydroxypropylation, a slurry is formed in the reaction mixture, in which the reactions take place in the surface layer of the starch granule. The first phase is carried out in an aqueous alkanol medium, in particular a medium containing 10 to 95% by weight of a lower alkanol, 5 to 90% by weight of water and 0.1 to 10% by weight of a catalyst.
The alkanol used is preferably methanol, ethanol, propanol, isopropanol, or n-butanol. The catalyst comprises a water-soluble, alkaline hydroxide compound such as sodium, potassium or ammonium hydroxide.
The hydroxypropoxylation is advantageously carried out at an elevated temperature. It has been found that a particularly preferred temperature, depending on the alkanol used, lies within the range from about 60 to 80xc2x0 C.
The hydroxypropoxylation is continued until the molar degree of substitution of the hydroxypropyl groups is 1 at the most. The reaction can be interrupted at the desired MS value by cooling the reaction mixture, by adding alkanol, by neutralizing the base or by adding propylene oxide into the initial mixture in an amount only which corresponds to the desired MS value.
An ester is prepared from the hydroxyalkylated starch in a manner known as such.
The preparation of the fatty acid esters of starch is performed, for instance, in the manner described in the following publications relevant in the field: Wolff, I. A., Olds. D. W. and Hilbert, G. E., The Acylation of Corn Starch, Amylose and Amylopectin, J. Amer. Chem. Soc. 73 (1952) 346-349, or Gros, A. T. and Feuge, R. O., Properties of Fatty Acid Esters of Amylose, J. Amer. Oil Chemists"" Soc 39 (1962) 19-24.
Starch acetates may be prepared by allowing the starch to react with acetanhydride in the presence of a catalyst. As the catalyst, for example a 50% sodium hydroxide is used. Even other known methods described in the literature for preparing acetates are suited for the preparation of starch acetate. By varying the amount of acetic acid anhydride, the amount of the base used as the catalyst, and the reaction time, starch acetates having different degrees of substitution can be prepared.
To cite an example of an advantageous thermoplasticized starch-based composition a composition may be mentioned containing hydroxypropyl starch acetate and plasticizer. Even this kind of composition has an elasticity or tensility which is at least 65 times better than that of plasticized starch acetate.
The hydroxyalkylated starch ester is plasticized by admixing it with a softener or plasticizer known as such. Therefore, the thermoplastic starch-based composition is advantageously made to contain 0.01 to 95% by weight, advantageously about 1 to 50% by weight and preferably about 10 to 40% by weight of a plasticizer. Any known plasticizers can be used, examples thereof including the following: triacetin, diacetin, monoacetin, triethyl citrate, tributyl citrate, acetyl triethyl citrate, acetyl tributyl citrate, dimethyl succinate, diethyl succinate, ethyl lactate, methyl lactate, fatty acid esters of glycerol, castor oil, olive oil, rapeseed oil, pine oil, dibutyl phthalate, diethyl phthalate, and mixtures thereof.
In addition to plasticizers, starch compositions may contain other additives and auxiliaries known as such within the field of polymer and plastics technology, such as lubricators, antistatic agents, colorants, pigments, fire retardants and reinforcing and filling agents. These additives and auxiliaries are present in an amount of about 1 to 95% of the weight of the composition, typically 10 to 50%. As regards the auxiliaries, particularly the following may be mentioned: waxes (e.g. alkylketene dimer wax (AKD) or beeswax, cf below), reinforcing agents (see below) and fillers, such as titanium dioxide, calcium carbonate, kaolin, aluminium hydroxide, sodium silicoaluminate, barium sulphate and zinc oxide.
Starch dispersion constitutes a particularly advantageous starch composition. In order to prepare it, a plasticized starch ether ester is added to water by the aid of a dispersion auxiliary, whereby the plasticized polymer melt can be dispersed in water in sufficiently fine particles in order to form a stable dispersion. Examples of dispersion auxiliaries include polyvinyl alcohol (PVA), particularly PVA having a weight-average molar mass of approximately 10,000 to 115,000. Other dispersion auxiliaries (protective colloids) include cationic starch and hydroxyalkyl starch which may be used separately or together with PVA. Furthermore, as additives or auxiliaries, the dispersions may contain alkylketene dimer (AKD) wax and beeswax.
Depending on the intended use, such a polymer dispersion can further be made to contain 0.01 to 30% by weight, preferably about 5 to 30% by weight of a cellulose ester, such as cellulose acetate, cellulose propionate or cellulose butyrate, or mixed esters thereof.
According to an embodiment the present dispersion compositions are prepared by dispersing the plasticized polymer melt in water using auxiliaries. In order to achieve plasticizing the biodegradable polymer is admixed with a plasticizer preferably at an elevated temperature in order to form a melt. On a small scale the plasticization can be carried out in e.g. a flask equipped with a reflux condenser and having efficient agitation. The temperature varies depending on the plasticizer used but is typically about 50 to 250xc2x0 C., preferably about 100 to 200xc2x0 C. On a larger scale, plasticization is advantageously performed in a melt-processing apparatus, such as an extruder.
The plasticized melt is dispersed in a liquid phase, usually water, using auxiliaries. Water is considered a particularly advantageous dispersion medium according to the invention but the invention can also be applied to various kinds of solvents.
According to another advantageous embodiment the dispersions are prepared by mixing together the HPS ester, the plasticizer, the dispersion auxiliaries and a part of the amount of water used for preparing the dispersion. The mixture is heated to obtain a paste-like composition whereafter the paste is dispersed in the remaining amount of water.
Plasticized starch ether ester compositions according to the invention may contain 1 to 95, advantageously about 5 to 45% by weight of fibrous material which serves to reinforce the material and simultaneously produces discontinuity interfaces enhancing the absorption of water. The fibrous material is added to the rest of the material at any stage, but not later than during the manufacturing process of the semi-finished product or the product. By way of exemplifying the biodegradable fibres which may be used in the invention, organic fibres, inorganic fibres and mixtures thereof may be cited. The fibrous material advantageously comprises fibres of a lactic-acid-based polymer (e.g. polylactide fibres), cellulose pulp (e.g. pine-based pulp), cellulose fibre material from corn (e.g. cellulose fibre material from the husk of barley corn), pentosan from corn (e.g. pentosan from the husk of barley corn), cotton linters, fibres of Abaca hemp, sisal fibres, ramie fibres, flax fibres, jute fibres, or biodegradable glass fibres. Of the organic fibre materials, cellulose fibres will improve the impact resistance values of the compositions by about 10 to 100% (as compared to an unstrengthened composition), other plant-based fibres by about 50 to 150%, and hydroxy acid polymer fibres by about 200 to 600%. The impact resistances of biodegradable glass fibres are improved by about 200 to 300%.